1. Field of the Invention
The present invention relates to a method for embedding and extracting a spatial domain blind watermark using a sample expansion, and more particularly, to a method for embedding and extracting a spatial domain blind watermark using a sample expansion which is applied for protecting a copyright information of a digital image or a digital video among digital contents.
2. Description of the Related Art
Watermarking is a kind of copyright protection technology in which a copyright holder's signal (watermark) that cannot be perceived by human being is embedded in digital data such as image, audio, video, etc., and the embedded watermark is then extracted to enable to claim the copyright. The watermark can be embedded in a logo style that can be visually perceived by human being, but due to easy deletion, deterioration of image quality, etc., many methods for embedding the watermark in a format that cannot be visually perceived by human being are being researched and used.
Then, in case that the watermark in the format that cannot be visually perceived is embedded, there are requested several conditions.
First is the invisibility that cannot be visually perceived by human being. In other words, the embedding of the watermark should not cause the quality of an original image to be lowered, and even though the quality lowering is caused, it should not be visually extracted.
Second is the robustness. In other words, although the embedded watermark is subject to various image processing or other's intentional attacks for deleting the watermark, it should be not deleted. There are various image processing technologies, for instance, loss and compression technology such as JPEG, image blurring, sharpening, analog/digital conversion, digital/analog conversion and the like, and the embedded watermark should be robust against these various attacks.
Third is the unambiguity of the watermark, which indicates a property in which the ownership of the embedded watermark is correctly discriminated. In other words, there should not occur problems in that non-embedded watermark is mistaken as if it is embedded or in that the embedded watermark is incorrectly perceived as the other's watermark.
Thus, embedding the copyright holder's inherent watermark that is not visually perceived in an image of the copyright holder, enables to solve the copyright problem when a third party claims the copyright of the image or illegally used the image, by extracting the embedded watermark of the original copyright holder. Also, when an image in which a purchaser's watermark is embedded, is sold and then the purchaser distributes the image illegally, the watermark extracted from the illegally distributed image enables to discriminate the purchaser. Thus, the watermark can be effectively used for the copyright protection of a digital image and the prevention of data copy.
The blind watermarking does not use the original image in order to extract the watermark, and is divided into a spatial domain watermarking and a frequency domain watermarking. Bender et al proposes a patchwork method that is one kind of the spatial domain watermarking methods that are widely spread. After that, many modifications of the patchwork method were proposed, and the invention employs the patchwork method basically.
In other words, the conventional patchwork method or a modification thereof selects a samples group and adds or subtracts a certain constant value to or from the sampled pixel. From the interpretation of the frequency domain, the above action results in adding a watermark component having a radio frequency component to an original image. Since the image compression such as JPEG contains an image processing procedure for removing the radio frequency component from the original image, if the watermark-embedded component is subject to the image compression attack, the image is severely damaged, so that there is generated a problem in that the embedded watermark is not extracted correctly.